Illumination systems for illuminating a space or object with a variable color are generally known. Generally, such systems comprise a plurality of light sources, each light source emitting light with a specific color, the respective colors of the different light sources being mutually different. The overall light generated by the system as a whole is then a mixture of the light emitted by the several light sources. By changing the relative intensities of the different light sources, the color of the overall light mixture can be changed.
It is noted that the light sources can be of different type, such as for instance TL lamp, halogen lamp, LED, etc. In the following, simply the word “lamp” will be used, but this is not intended to exclude LEDs.
By way of an example of a variable color illumination system, an illumination system in a home, office, shops, restaurants, hotels, schools, hospitals, etc. is mentioned. The use of colors and color variation, in conjunction perhaps with seasons and/or events, may be beneficial for attracting attention of customers, for influencing the mood of customers, for creating a certain atmosphere, etc.
Typically, an illumination system comprises three lamps of single color, which will also be indicated as the primary lamps generating primary colors. Usually, these lamps are close-to-red (R), close-to-green (G), close-to-blue (B), and the system is indicated as an RGB system. For each lamp, the light intensity can be represented as a number from 0 (no light) to 1 (maximum intensity). A color point can be represented by three-dimensional coordinates (ξ1, ξ2, ξ3), each coordinate in a range from 0 to 1 corresponding in a linear manner to the relative intensity of one of the lamps. The color points of the individual lamps can be represented as (1,0,0), (0,1,0), (0,0,1), respectively. These points describe a triangle in the color space. All colors within this triangle can be generated by the system by suitably setting the relative intensities ξ1, ξ2, ξ3 of the respective lamps. More particularly, each color within this triangle can be obtained in one way only, as a unique combination of the relative intensities ξ1, ξ2, ξ3 of the respective lamps.
It is also possible that an illumination system has four lamps with mutually different colors, i.e. four primaries. As a fourth lamp, a white lamp may be used, which will improve the light output for colors close to the white point, and which is typically used for systems that are mainly used for generating white light. It is also possible that an additional color is used. For instance in the case of fluorescent lamps, it is known to add a yellow lamp to widen the color gamut in the yellow region. Also in the case of fluorescent lamps, it is known to add a red neon lamp to compensate for the unsaturated red of fluorescent lamps; this will also widen the color gamut in the yellow region. In the case of a system with LEDs, it is known to add an amber lamp in order to improve the color rendering index.
In the case of a four-lamp system, the relative intensities of the respective lamps can be written as ξ1, ξ2, ξ3, ξ4. A complication in such case is that most colors (or even all colors) can be obtained not as a unique combination of the four relative intensities ξ1, ξ2, ξ3, ξ4: many such combinations are possible for resulting in the same mixed color.
Thus, if a user selects a certain desired output color, a problem is to find a set of relative intensities ξ1, ξ2, ξ3, ξ4 of the primary lamps. In prior art, there are several different approaches for solving this problem. For instance, it is possible to set one of the primaries to zero, so that the problem translates to a three-primary problem again. Or, it is possible to fix the ratio between the relative intensities of two primaries, to again obtain a problem with three variables. US-2005/00833ξ1-A1 discloses a complicated method based on defining several color triangles.